1. Field of the Invention
The present invention relates to a blood-pressure-waveform monitoring apparatus which continuously displays a waveform representing blood pressure of a living subject.
2. Related Art Statement
During a surgical operation, or in an intensive care unit, a blood-pressure-waveform monitoring apparatus may be used to continuously monitor blood pressure of a patient. The blood-pressure-waveform monitoring apparatus includes a blood-pressure-waveform detecting device which continuously detects a blood-pressure waveform from the patient, and a display device which displays the blood-pressure waveform detected by the blood-pressure-waveform detecting device. The blood-pressure-waveform detecting device may be one which employs a tonometry in which a pressure-pulse-wave sensor is used to detect a blood-pressure waveform, or one which employs a catheter method in which a catheter is used to detect a blood-pressure waveform. In the tonometry, the pressure-pulse-wave sensor is fixed above an artery, such as a radial artery, that is considerably near to skin, and is pressed with an appropriate force against the artery via the skin to detect a pressure pulse wave that is produced by pulsation of the artery in synchronism with heartbeat of the patient. The thus detected pressure pulse wave represents relative change of blood pressure in the artery, and therefore represents a blood-pressure waveform. Meanwhile, the catheter-type pressure-waveform detecting device requires a person to invasively insert the catheter into an artery of the patient so that a pressure-converting element in the catheter can directly detect blood pressure in the artery.
Thus, the tonometry-type pressure-waveform detecting device can accurately measure relative change of the intraarterial blood pressure, but cannot accurately measure absolute blood pressure because of visco-elasticity of the skin or subcutaneous tissue located above the artery. Hence, it is needed to wind a cuff around an upper arm of the patient, measure a blood pressure of the upper arm of the patient using the cuff, and calibrate the relative change of intraarterial blood pressure based on the thus measured upper-arm blood pressure. This calibration is periodically performed since the pressure-pulse wave sensor may be moved out of position. In addition, when the blood pressure detected by the pressure-pulse-wave sensor is judged as being abnormal, a blood-pressure measurement using the cuff may be started to obtain a more reliable blood-pressure value. Meanwhile, the catheter-type pressure-waveform detecting device usually requires a person to insert the catheter into a considerably distal portion of a peripheral artery of a patient and, in an emergency, a cuff may be needed to be wound around an upper arm of the patient so as to obtain a central-side blood-pressure value of the patient.
As described above, in the continuous blood-pressure monitoring using the tonometry-type pressure-waveform detecting device, not only the pressure-pulse-wave sensor is worn on a patient, but also the cuff is worn on an upper arm of the patient. In many cases, concurrently with the continuous blood-pressure-waveform monitoring, the patient receives an intravenous drip. In those cases, however, the cuff or the pressure-pulse-wave sensor cannot be worn on the arm into which the drip tube is inserted, because, if the cuff presses the upper arm, the veins are occluded and the drops cannot smoothly flow into the veins, or because the pressure-pulse-wave sensor or the cords of the sensor interfere with the drip tube.
Thus, in many cases, the pressure-pulse-wave sensor is worn on the wrist of the arm around which the cuff is wound. In those cases in which the pressure-pulse-wave sensor and the cuff are worn on the same arm, the pressure-pulse-wave sensor that is located on the downstream side of the cuff cannot detect an accurate blood-pressure waveform during a blood-pressure measurement using the cuff. Therefore, the display device is stopped from displaying the inaccurate blood-pressure waveform, or otherwise is allowed to display the blood-pressure waveform having an abnormal shape. A blood-pressure measurement using the cuff may take more than one minute when an abnormally high blood pressure or an arrhythmia occurs to the patient. That is, although the blood-pressure-waveform monitoring apparatus is used in those cases in which it is medically needed to continuously monitor a blood-pressure waveform of a patient, each blood-pressure measurement using the cuff interrupts the continuous blood-pressure-waveform monitoring and thereby disadvantageously reduces the significance of use of the monitoring apparatus.
Likewise, the catheter-type blood-pressure-waveform monitoring apparatus requires, in many cases, the catheter and the cuff to be worn on the same arm of a patient. Therefore, like the tonometry-type blood-pressure-waveform monitoring apparatus, the catheter-type blood-pressure-waveform monitoring apparatus suffers the problem that each blood-pressure measurement using the cuff disadvantageously interrupts the continuous blood-pressure-waveform monitoring.
It is therefore an object of the present invention to provide a blood-pressure-waveform monitoring apparatus which can display an accurate blood-pressure waveform even during a blood-pressure measurement using a cuff.
The above object has been achieved by the present invention. According to the present invention, there is provided an apparatus for monitoring a blood-pressure waveform representing a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a first portion of the subject; a blood-pressure determining device which determines a blood pressure of the subject based on a signal obtained while a pressure in the cuff is changed; a blood-pressure-waveform detecting device which is adapted to be worn on a second portion of the subject and continuously detects the blood-pressure waveform representing the blood pressure of the subject; a display device which displays the blood-pressure waveform detected by the blood-pressure-waveform detecting device; a cuff-using-blood-pressure-measurement starting means for operating, when a prescribed blood-pressure-measurement-starting condition is satisfied, the blood-pressure determining device to determine the blood pressure of the subject; a pulse-wave detecting device which is adapted to be worn on a third portion of the subject that is not located on a downstream side of the first portion around which the cuff is wound, and which detects a pulse wave from the third portion of the subject; an estimated-blood-pressure-waveform determining means for continuously determining, based on the pulse wave detected by the pulse-wave detecting device, an estimated blood-pressure waveform representing an estimated blood pressure of the subject, according to a predetermined relationship between blood-pressure waveform and pulse wave; and a waveform-displaying control means for operating, when the blood-pressure waveform detected by the blood-pressure-waveform-detecting device is not abnormal, the display device to display the blood-pressure waveform, and operating, when the blood-pressure waveform is abnormal, the display device to display the estimated blood-pressure waveform in place of the blood-pressure waveform.
Since the pulse-wave detecting device is worn on the portion of the subject that is not located on the downstream side of the portion around which the cuff is wound, the pulse wave detected by the pulse-wave detecting device is not influenced by the inflation of the cuff to press the portion around which the cuff is wound. Therefore, the estimated blood-pressure waveform determined by the estimated-blood-pressure-waveform determining means based on the pulse wave actually detected by the pulse-wave detecting device according to the predetermined relationship between blood-pressure waveform and pulse wave, is not influenced by the inflation of the cuff. On the other hand, if the pressure-waveform detecting device is worn on a portion of the subject that is located on the downstream side of the cuff, the blood-pressure waveform detected by the pressure-waveform detecting device becomes abnormal because of the inflation of the cuff. In this case, however, the waveform-displaying control means operates, when the blood-pressure waveform detected by the pressure-waveform detecting device is abnormal, the display device to display, in place of the blood-pressure waveform, the estimated blood-pressure waveform determined by the estimated-blood-pressure-waveform determining means. Thus, the present apparatus displays an accurate blood-pressure waveform even during each blood-pressure measurement using the cuff.
According to a preferred feature of the present invention, the pulse-wave detecting device comprises a photoelectric-pulse-wave detecting device which is adapted to be worn on the third portion of the subject, emits a light toward the third portion, and detects a photoelectric pulse wave from the third portion. The photoelectric-pulse-wave detecting device is small-sized and light-weighted. Therefore, even in the case where a drip tube is inserted in a blood vessel of a first arm of the subject that is different from the other or second arm of the subject on which the cuff is worn, the photoelectric-pulse-wave detecting device can be worn on the first arm, without interfering with the drip tube.
According to another preferred feature of the present invention, the waveform-displaying control means judges that the blood-pressure waveform is abnormal, when the pressure of the cuff is not lower than a prescribed lowest pressure at which the cuff would at least partly block a flow of blood through the first portion of the subject, and thereby operates the display device to display the estimated blood-pressure waveform. In the case where an estimated blood-pressure waveform is displayed when an abnormality of the blood-pressure waveform is found directly from the blood-pressure waveform itself, it may be difficult to find the abnormality of the blood-pressure waveform. However, according to this feature, when an abnormality of the blood-pressure waveform is caused by the inflation of the cuff, the estimated blood-pressure waveform is displayed with reliability in place of the blood-pressure waveform being abnormal.